Problem: On the first 3 exams of her physics class, Nadia got an average score of 88. What does she need on the next exam to have an overall average of 90?
Let her score on the next exam be $x$ The sum of all of her scores is then $3 \cdot 88 + x$ The same sum must also be equal to $4 \cdot 90$ Solve: $x = 4 \cdot 90 - 3 \cdot 88 = 96$.